In the full term neonate, the ductus arteriosus constricts rapidly after delivery in order to separate the pulmonary from the systemic circulations. Failure of the ductus to close after birth results in significant morbidity: chronic lung disease and necrotizing enterocolitis. Permanent closure of the ductus arteriosus is accomplished by extensive remodeling of the vessel wall: neointimal mounds that occlude the lumen are formed by proliferating luminal endothelial cells and migrating smooth muscle cells from the muscle media; ductus arteriosus constriction produces profound hypoxia of the inner vessel wall which is essential for initiating the anatomic remodeling. The ductus arteriosus in preterm infants is susceptible to prolonged persistent patency after birth because it fails to constrict as tightly as the mature ductus and is more resistant to developing hypoxia during postnatal constriction. The studies proposed in this application will examine the reason for these differences. They will examine the signaling pathways that play a role in ductus constriction and how they differ between the mature and immature ductus. They will examine the major endogenous vasodilator of the ductus, PGE2, to see how PGE2 receptors are regulated. They will examine the control of vasa vasorum perfusion of the ductus as a mechanism for producing hypoxia. They will examine the role of vascular endothelial growth factor in producing the anatomic changes observed in anatomic remodeling. They will study the ductus arteriosus using in vivo models in the fetus and newborn, in vitro models using ductus arteriosus in organ culture, and in vitro models using isolated endothelial and smooth muscle cells in culture. They will use displacement-binding assays, immunohistochemistry, Western blot, Northern blot, Real Time PCR, and electrophoretic mobility shift assays to examine changes in mRNA and protein expression and receptor binding function. They will measure isometric tension in vitro. They will measure blood flow in vivo using the fluorescent microsphere technique. These studies should increase our understanding of what initiates and sustains the process of ductus closure after birth and why it does not occur in the preterm infant.